The present invention relates to the field of sonar antennas, in particular those intended to be installed on an underwater vehicle in order to constitute a mounted head of said vehicle. It relates in particular to a sonar antenna capable of operating in different frequency bands.
A sonar antenna generally consists of a large number of transducers Each transducer, for example a receiving transducer, consists of an element for converting the pressure energy that propagates in the liquid medium into electrical energy that can be processed by electronic means. In an underwater antenna such as that used in sonars for torpedoes, the transducers are arranged alongside one another on an acoustically transparent material which ensures energy transfer; they are moreover fastened mechanically, in a more or less complex manner, onto a support capable of withstanding compression forces.
One example that may be cited is French Patent No. 2603761, which describes a sonar antenna constituting the mounted head of an underwater vehicle, said antenna having a core which is a block of rigid syntactic foam that is resistant to immersion-related hydrostatic pressure, said block having receptacles in each of which is placed an electro-acoustic transducer, the outer surface of which is flush with the outer surface of said block. This antenna has a sealed casing made of an acoustically transparent material, which is overmolded around the block and the transducers and which has a hydrodynamic tapered profile which extends that of the vehicle hull, and means for fastening onto the outer surface of the front end of the vehicle hull. Underwater antennas can operate exclusively passively or actively, or can have the capability for simultaneous active and passive operation.
In all cases, the antenna possesses an optimum operating frequency which is characterized by the fact that sampling of the sensitive area, in other words the number of transducers per unit area, is suitable; and, for active antennas, that each transducer can convert electrical energy into mechanical energy with the best possible efficiency. In an effort to meet the needs of designers of torpedo seeker heads, who are demanding more and more frequency agility for the sonar, experiments with "wide-band" active antennas have led to the design, by more or less complex means, of transducers whose efficiency remains at a level sufficient for a frequency excursion on the order of 30% of the optimum frequency. Factors which, however, still cannot be varied as a function of frequency are on the one hand the dimension of the transducer head mass which constitutes the coupling element between the medium and the energy conversion material, said dimension establishing the area sampling; and on the other hand the dimensions of the constituent parts of the transducers, which establish the optimum operating frequency.
The "dimension" of a transducer head mass is understood to mean the diameter of the head mass if it is in the shape of a flattened cylinder, or the length of one side if it is in the shape of a rectangular parallelepiped with a square base.
For physical reasons relating to energy transmission and reduced interaction among the transducers, the designer arranges the elements at distances that remain close to half the wavelength that is propagating in the medium.
British Patent No. 2,077,552 describes a sonar antenna which can be used to search for shoals of fish or sandbanks, and which has transducers that emit at two frequencies .nu..sub.1, and .nu..sub.2, respectively 55 kHz and 130 kHz, the dimension of the head mass of each transducer being substantially equal to half the wavelength .lambda..sub.1 corresponding to frequency .nu..sub.1. With an antenna of this kind, however, efficiency drops considerably when the transducers are operating at frequency .nu..sub.2. Moreover an antenna of this kind cannot be used in a torpedo seeker head because of its very low directivity.